Light shutter systems such as, for example, stereoscopic systems switching between opaque and transmissive optical states to alternately transmit left-and right-eye images to a viewer preferably have high contrast ratios. A maximum contrast ratio can be achieved by providing maximum output light extinction in the opaque state to block an image from the viewer's eye and maximum light transmission in the transmissive state to convey an image to the viewer's eye.
Light shutters have been proposed that include variable optical retarders positioned between crossed linear polarizers. The variable optical retarders switch ideally between zero and half-wave retardation states. In the ideal case, linearly polarized incident light propagates unaffected through the variable optical retarder in its zero retardation state and is completely blocked by the output linear polarizer. Linearly polarized incident light propagating through the variable optical retarder in its half-wave retardation state undergoes a 90.degree. rotation of polarization direction and is completely transmitted by the output linear polarizer. In this ideal light shutter, the zero retardation state of the variable optical retarder corresponds to an opaque state of the shutter, and the half-wave retardation state of the variable optical retarder corresponds to the transmissive state of the shutter.
Practicably realizable variable optical retarders are often incapable of achieving true zero retardation values and, therefore, have a non-zero amount of residual retardation or birefringence when it is desired that the shutter be in the opaque state. This residual retardation elliptically polarizes the light propagating through the variable optical retarder in the opaque state. As a consequence, light exiting the variable optical retarder cannot be entirely blocked by the output linear polarizer, and the extinction of light in the opaque state is not as high as desired. This imperfect extinction of light in the opaque state lowers the contrast ratio, which is defined as the ratio of the light intensity transmitted in the transmissive and opaque states of the shutter.
One way of reducing residual birefringence is to apply a very high voltage across the liquid crystal cell and thereby introduce a high intensity electric field within the cell. The use of high drive voltages is undesirable, however, because it entails the increased expense of providing a high voltage circuit that is capable of driving the capacitive load presented by the cell and creates possible reliability problems stemming from a consequent increase in power dissipation.
U.S. Pat. No. 4,767,190 of Dir et al. describes a liquid crystal image bar having an enhanced contrast ratio. The Dir et al. image bar includes a birefringent liquid crystal cell positioned between an elliptical analyzer and a linear polarizer to produce opaque and transmissive optical states. The elliptical analyzer comprises a tilted polarizer and a quarter-wave plate that cooperate to compensate for elliptical polarization introduced by the residual birefringence of the liquid crystal cell when it is excited to its field-aligned state. The effect of the residual birefringence is to reduce light extinction in the opaque state. The elliptical analyzer provides maximum light extinction in the opaque state at the possible expense of a quantity of light loss in the transmissive state.
Dir et al. notes, in column 9 beginning at line 51, that the contrast enancement achieved with the described quarter-wave plate approach results in nearly doubled response times. Such an increase is often undesirable and in many applications is unacceptable.
U.S. Pat. No. 4,884,876 of Lipton et al. describes an achromatic stereoscopic system using a linear polarizer, an elliptical analyzer, and a liquid crystal cell of the same construction taught by Dir et al. to provide alternating left- and right-eye image scenes to a viewer. Lipton et al. states that judicious alignment of the transmission axes of the linear and elliptical polarizers and the rub axis of the liquid crystal cell achieves an optimal dynamic range between optical states. Lipton et al. advises that an optimal dynamic range may be less than the maximum dynamic range.
Applicant believes this compromise characterizing the design of the Lipton et al. system stems from the inability of Lipton et al. to provide a system that can both minimize light transmission in the opaque state and maximize light transmission in the transmissive state.